As used herein the term "phase" is intended to refer to separate immiscible liquid phases such as oil and water as well as liquid and gas phases. As used herein the term "multiphase" is intended to refer to a mixture including at least two such phases. The term "slug flow" refers to the slug or plug flow regime of the kind that occurs in pipelines from oil wells carrying the three-phase mixture of oil, water and gas. These flows are characterised by intermittent but well defined slugs of water, oil and gas mixtures which essentially fill the cross section of the pipeline. These slugs are separated by what are termed films, which are portions of the flow in which the pipeline is substantially filled with gas accompanied by a small amount of mixed water and oil.
The mass flow rate measurement of the oil, water and gas from individual oil wells is important for better reservoir management, better production allocation, and optimisation of total oil production over the field life. Normally, the required accuracy of determination of mass flow of each phase is 5%.
Additionally, there is often a need to measure the relative concentrations of oil and water in a flow after separation of the gas and some of the water. This measurement can present considerable practical difficulty particularly where the densities of the oil and water are the same or similar.
Current practice for the measurement of mass flowrate of the phases of oil well flows is to periodically physically divert the well output to a test separator. After separation the flow rate of each component is measured with conventional devices such as orifice or turbine flow meters. There are several inherent disadvantages associated with this technique. Firstly, accurate measurement requires stabilised well flow which can take some time to establish. Often, testing the output of a single well may take a whole day. In addition, the physical size of the separator and associated equipment occupies significant space which can lead to increased costs on off-shore platforms. Finally, in practice it is not feasible to provide each well with its own test separator system and often many wells share a common facility. Continuous monitoring of the output of each well is therefore not possible.
Various techniques have been suggested for on-line mass flow measurement of multiphase mixtures. Most depend on determination of the concentration of one or more of the phases coupled with a determination of either the mean velocity of one or more of the phases or the total mass flow of the mixture. Concentration measurement by capacitance is described in a paper entitled "On-line measurement of oil/water/gas mixtures using a capacitance sensor" by Beck M. S. Green R. G., Hammer E. A. and Thorn R., Measurement 3 (1) 7-14 (1985). Measurement of component concentration using a dual energy gamma-ray transmission technique has also been described by the following:
Fanger U., Pepelnik R. and Michaelis W.--Determination of conveyor-flow parameters by gamma-ray transmission analysis, pp. 539-550 in Nuclear Techniques and Mineral Resources 1977, IAEA, Vienna, 1977. PA0 Michaelis W. and Fanger H. U.--Device for determining the proportions by volume of a multiple-component mixture, U.K. Patent Application GB2083908 A, 1982. PA0 Abouelwafa M. S. A. and Kendall E. J. M.--The measurement of component ratios in multiphase systems using gamma-ray attenuation, J. Phys. E.: Sci. Instrum, 131 341-345 (1980). PA0 Kendall E. J. M.--Gamma-ray analysis of multicomponent material, U.K. Patent Application GB 2088050 A, 1982. PA0 Tomada T., Komaru M., Badono S., Tsumagari K. and Exall D.--Development of gamma-ray oil/water/gas fraction meter for crude oil production systems, Paper presented at the International Conference on Industrial Flow Measurement On-shore and Off-shore, 22-23/9/87, London.
Microwave measurement of phase concentration is also known from U.S. Pat. No. 4,301,400. Neutron inelastic scatter techniques have also been used.
Energy transmission techniques for measurement of phase concentration, in which the radiation source is on the opposite side of the pipe from the radiation detector, have the advantage over reflection or scatter techniques of being sensitive to fluid volumes right across the pipe and hence minimising the errors in determination of flow rate caused by heterogeneity of the three phase mixture. Transmission measurements can be based on the use of one, or the combination of more than one, of the following: gamma-rays, neutrons, microwaves, infrared or ultrasonics. Some transmission measurements require long radiation path lengths in oil and water to obtain adequate sensitivity for the determination of the oil and water volume and mass fractions. For this reason transmission techniques suffer loss of sensitivity in slug flow measurement since the liquid path length in the film between slugs is comparatively short.